The leakproofness, low oxygen content and adequately low storage temperature of a package are the most important requirements for many packaged foodstuffs. When a protective gas package leaks, the protective gas travels out of the package, oxygen, damaging to the preservation of the quality of many products, gets into the package and the additional preservation provided by the original gas mixture of the package is lost. The passage of oxygen into the package is damaging also in the case of vacuum packages. In addition to the wholeness and storage temperature of a package the high quality of a raw material used is an essential factor for maintaining organoleptic and microbiological quality particularly with uncooked products. As a result of the microbe activity occurring during the deterioration of a product, numerous amounts of evaporative compounds and compounds remaining in the product are generated whose quality and amounts are greatly affected by the nature and chemical composition of the foodstuff as well as by the microbe causing deterioration. These generated compounds affect on one hand the organoleptic quality of the foodstuff and on the other hand they function as indicators of microbiological quality of the foodstuff. The composition of the compounds generated by deterioration depends on the foodstuff, and for example in case of the deterioration of poultry the generation of different types of sulphur compounds (for example hydrogen sulphide, dimethyl sulphide, dimethyl disulphide) is typical.
Known applications associated with RF-reading techniques are for instance burglar alarms and remote sensors (RFID).
The burglar alarms used in these applications are either destroyed or deactivated in a controlled manner. Remote sensors for their part are suitable only for recognition of a product or for saving a piece of information into the memory of a remote sensor. Burglar alarms and remote sensors do not have the ability to indicate cumulative events such as deterioration inside a foodstuff package.
The color change of previously described indicators reacting to compounds generated in deterioration or to oxygen content of the package is visible. The primary function of visible indicators is to help the consumer evaluate the quality of the product at the time of purchase or at home. On the other hand on the part of wholesale and retail sales it would be efficient to verify the wholeness of the package and the quality of the product already before a consumer purchases it.
From WO 95/33991 is known a solution in which an indicator contains electronics and typically a screen that is integrated into the indicator. Alternatively, the indicator may have an output for sending a signal galvanically to the exterior measuring device. Such an indicator provided with its own screen is inevitably an expensive solution. Reading by an exterior device implemented by a galvanic connection is on the other hand quite an awkward way to obtain information from individual foodstuff packages.
From e.g. U.S. Pat. No. 5,443,987, WO 9821120, EP patent 0666799 and WO 9904256 are known indicators in which a change in color or outward appearance occurs when the product deteriorates.
From U.S. Pat. No. 5,663,072 is known a solution in which by adding suitable chemicals the condition of a meat package can be evaluated by the changes of the absorption or reflective properties of the package itself upon exposure to electromagnetic radiation. The suitability of chemicals for use in foodstuffs may create problems and additionally the manner of measurement is quite inexact.
None of the hydrogen sulphide sensors described above is readable by RF-techniques nor are they suitable for placement in a foodstuff package and being read without breaking or touching the package. On the other hand the publications mentioned above also do not present application of RF-reading technology for measuring the quality of the packaged foodstuff.
From application No. PCT/FI02/00911 is known a solution in which an indicator is placed inside a package, wherein the indicator is an LC (inductor-capacitor) circuit to which a sensor element is connected. The electrical properties of the sensor element change cumulatively as a result of deterioration of the product. The sensor reacts either directly to deterioration of the product or to evaporative compound resulting from deterioration or to oxygen gas that has leaked into the package. In this solution the sensor element is coupled by bonding or electricity-conducting glue to the LC resonator such that electrical current induced to the circuit flows through the sensor element. Deterioration, an evaporative compound resulting from it or oxygen will cause corrosion in the sensor element, which is why the sheet resistance of the sensor increases. The change in the properties of the element affects either the loss resistance of the LC resonator or through it the quality factor of the circuit, the so-called Q-factor, or the sensor element changes the capacitance or the inductance of the circuit. What is essential in this solution is that the entire indicator formed by the LC circuit and the sensor element is placed inside the package and the sensor element is coupled to the resonance circuit essentially galvanically, i.e. by bonding or gluing. The indicator is read from outside the package by a reading device. It generates an alternate magnetic field whose frequency is typically wiped through the resonance frequency of the LC resonator. In this way the reading device measures the resonance frequency and the Q-factor of the LC resonator. The measurement result is proportional to the deterioration of the product or to the amount of oxygen in the package.
In this method the disadvantage is that the manufacture of the indicator formed from an LC resonator is relatively expensive. In particular, the attachment of the sensor element by either bonding or gluing with electricity-conducting glue is not inexpensive. Bonding is an expensive technique. It is not an adequately cost-effective means to be used in cheap mass production. Attachment by electricity-conducting glue for its part would cause a too high contact resistance. It has become apparent that the contact impedance of the sensor element and the LC resonator should be in the range of 0.2 Ohms. In practice, achieving such low total impedance is technically difficult and expensive.
In the publications “Design and application of a wireless, passive, resonant-circuit environmental monitoring sensor”, Sensors and Actuators A 93, 2001, 33-43 and “Monitoring of bacteria growth using a wireless, remote query resonant-circuit sensor: application to environmental sensing”, Biosensors&Bioelectronics 16, 2001, 305-312 an indicator based on an LC resonator is also described. In these publications the method is applied to monitoring an environment and indicating bacteria growth.